Cohort profile: the Lothian Birth Cohorts of 1921 and 1936.

This cohort profile describes the origins, tracing, recruitment, testing and follow-up of the University of Edinburgh-based Lothian Birth Cohorts of 1921 (LBC1921; N = 550) and 1936 (LBC1936; N = 1091). The participants undertook a general intelligence test at age 11 years and were recruited for these cohorts at mean ages of 79 (LBC1921) and 70 (LBC1936). The LBC1921 have been examined at mean ages of 79, 83, 87 and 90 years. The LBC1936 have been examined at mean ages of 70 and 73 years, and are being seen at 76 years. Both samples have an emphasis on the ageing of cognitive functions as outcomes. As they have childhood intelligence test scores, the cohorts' data have been used to search for determinants of lifetime cognitive changes, and also cognitive change within old age. The cohorts' outcomes also include a range of physical and psycho-social aspects of well-being in old age. Both cohorts have a wide range of variables: genome-wide genotyping, demographics, psycho-social and lifestyle factors, cognitive functions, medical history and examination, and biomarkers (from blood and urine). The LBC1936 participants also have a detailed structural magnetic resonance imaging (MRI) brain scan. A range of scientific findings is described, to illustrate the possible uses of the cohorts.